Experimental Petrology of Planetary Materials

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WalkerEAR-0207546The general aim of this project is to understand planetary differentiation by igneous processes. Laboratory experiments are used to simulate those processes. The major expression of planetary differentiation is the separation of core and mantle. The highly siderophile elements of the Pt group are a key suite of elements for understanding the formation and evolution of Earth's core. Their geochemical partitioning between silicate and metal has been poorly understood because of technical problems in the interpretation of the dispersed metal nuggets found in experimental silicates equilibrated with metal at the low pO2 relevant to the core formation process. New experiments show that for Pt the nuggets are not artificial contaminants but are Pt recovered from solution during the incomplete quenching process. This preliminary result makes many of the enigmas of the high abundance of Pt in the Earth's mantle less perplexing. Pt is orders of magnitude more soluble in silicate than conventionally thought. We will continue to explore this result with time and temperature series studies. If verified, we will extend the study of nugget formation to Re and Ir, two elements of the group also known to also have their geochemical behavior clouded by nugget formation in experimental work. Additional projects to be undertaken include the completion of studies of the volumes of garnets across the solid solution series grossular-pyrope at simultaneous high-P and T by synchrotron XRD. This study has shown some interesting anomalies compared to conventional practices used to make these measurements. The results are relevant to the mixing and melting properties of garnets in mantle peridotite. Garnet is one of the important repositories of fusible components that liberate basalt upon deep peridotite partial melting. A related study will use the NaCl-KCl system to model the behavior of a solvus in a binary solution at high-P,T. We will also measure the melting points of B1 and B2 RbCl to ~30 kbar to investigate the bond weakening which seems to take place on conversion of phases to higher density, higher coordination number polymorphs. This study is relevant to expected convective style changes across phase change boundaries in the Earth. Finally, we hope to complete the data reduction of epithermal neutron resonance pyrometry measurements of the effect of P on thermocouple emf. This subproject is relevant to the calibration of widely used thermocouple pyrometry in use at high pressure.